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- TestKing question
- Piotr Witkowski
March 9, 2005, 10:36 pm
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Iam learning to CCNA exam (648-801) from TestKing
Below is question, and i consider that a right answer is B but TestKing
suppose that is a C, what do You think about that ?
The corporate head office of TestKing has a teleconferencing system that
uses a VOIP (voice over IP) system that uses UDP datagrams to send and
receive voice data. If for whatever reason the network connection is
compromised, what would happen if the datagrams would arrive to their
destination out of sequence?
A. UDP will send an ICMP Information Request to the source host.
B. UDP will pass the information in the datagrams up to the next OSI layer
in the order that they arrive.
C. UDP will drop the datagrams.
D. UDP will use the sequence numbers in the datagram headers to reassemble
the data in the correct order.
E. UDP will not acknowledge the datagrams and wait for a retransmission of
In my opinion UDP doesnt know that received a segment out of sequence,
because UDP segments hasn't any field in header with sequence number or
something similary, so UDP pass this segment to upper layers and them should
have a detecet error mechanism.
Re: TestKing question
yes, UDP has no sequence field in its header. This may not be the case
for other L4 protocols such as RTP (often used for VoIP) - they may
have been thinking of this.
It's more common to lose packets than to get a lot out of order. In
this scenario, it seems to me there's a good possibility they are using
a routing protocol that supports load sharing (RIP or IGRP).
Re: TestKing question
On Wed, 9 Mar 2005 14:36:35 +0100, "Piotr Witkowski"
I would say that B is the right answer. Out-of-sequence is also part
of the QoS problems with VoIP.
They also talk specifically about UDP and not about RTP. RTP would be
the next higher layer OSI protocol. If RTP has any sequence numbers
it's RTS's job to deal with it, not UDP.
As for testking, don't believe them more as far as you can throw a
rock at them. I guess the only reason Cisco doesn't sue them is
because a lot of answers are incorrect. The quality really sucks and
it's not worth the 60-or-so bucks you have to pay for it.
Better get a decent book and use your own healthy reasoning. This will
also make sure you pass the exam.
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